1. Field of the Invention
The present invention relates to impact printers, and more particularly, to apparatus for affecting the flight time of a print hammer in a hammer bank assembly.
2. Description of the Prior Art
In high speed, moving-type impact printers of the kind typically employed in data processing systems, a separate print hammer is usually situated at each print position. There are typically 132 print positions (and hammers) across a print line. Hammer bank assemblies for this purpose are described in U.S. Pat. No. 3,643,595 to Helms et al. and U.S. Pat. No. 3,983,806 to Ishi, and in pending U.S. patent application Ser. No. 065,766, filed Aug. 20, 1979, all assigned to Dataproducts Corporation, the assignee of the present invention.
Each print hammer of such hammer bank assemblies typically has a flat, electrically conductive coil disposed in a generally rigid housing or body. The body has an impact tip at one end and is supported by a pair of crossed spring wires at the other end. Each coil is an electromagnet which is energized by passing a current through the coil. The springs provide electrical contact to the coil, and aid in restoring the hammer to its rest position subsequent to impact.
In a hammer bank assembly, each hammer is situated between a pair of stationary flat permanent magnets. When a current flows through the coil of a print hammer, the resultant magnetic field in the coil interacts with the field of the adjacent magnets, resulting in a force which propels the hammer towards the type characters and the medium being printed. The type characters are usually carried in relief on a closed loop band (FIG. 2) which rapidly passes in front of the hammer bank assembly. A particular print hammer is energized when the desired character to be printed passes in front of the print position for that print hammer. The basic configuration and operation of such a printing hammer apparatus is set forth in U.S. Pat. No. 3,279,362 to Helms, also assigned to Dataproducts Corporation. Other hammers are shown in U.S. Pat. Nos. 3,279,364 to Helms, 3,643,595 to Helms, et al., and 3,983,806 to Ishi.
To insure proper registration between the print hammer and the characters to be typed on the character band, it is important to be able to control the flight time of the print hammers (i.e., the time it takes for a hammer to move from the rest position to the impact position after energization). The flight time of a particular print hammer is a function of the magnetic flux produced by the adjacent stationary permanent magnets and the flux produced by the coil embedded in the print hammer itself.
The magnetic flux produced by the stationary permanent magnets can vary as a function of the temperature of the magnets. Specifically, the magnets can become heated by repeated energization and by the operation of adjacent print hammers. As a particular magnet grows hotter, its magnetic flux can decrease. As a result, the flight time of the print hammers adjacent that magnet will be altered as the magnet grows hotter. Furthermore, the permanent magnets of the hammer bank assembly may not all be heated at the same rate, with the result being that some magnets may become hotter than others. Thus, the variation of the individual flight time for each print hammer may be different than for other hammers. This non-uniform variation can result from some print hammers being utilized more often than others. For example, in check printing operations, only print hammers to one side are utilized (since the check does not extend across the entire length of the hammer bank assembly). Accordingly, the flight times of the print hammers on the side of the hammer bank used to print the checks would tend to change relative to the unused print hammers. This variation in the flight time of the print hammers can result in misregistration between the hammer and moving characters, thus causing the characters being printed to be poorly formed and thereby reducing the quality of the printing operation.
In order to avoid the temperature induced variation of the print hammer flight times, prior art impact printers have utilized permanent magnets, such as rare or earth alnico magnets, which have relatively little variation of magnetic flux with changes in temperature. However, these low flux variation magnets tend to be quite expensive and their use results in a significant increase in the cost of the hammer bank assembly.
Furthermore, it has been found that the magnetic flux provided by each of the coils embedded in the print hammers also varies with temperature. As a coil is repeatedly energized, the temperature of the coil and hence the resistance of the coil will rise. With a constant applied voltage and increasing resistance, the current through the coil will decrease and the flux produced by the coil will also decrease. This will also slow down the flight time of the print hammers.
In order to maintain the flux produced by the coils relatively constant, constant current sources are typically utilized to energize the coils. These constant current sources often require expensive custom integrated circuits to supply a constant current to the coils.